1. Field of the Invention
The present invention relates to a method and apparatus for generating microwaves and, more particularly, the present invention relates to a microwave source and method for generating a train of bipolar pulses.
2. Description of the Related Art
High power microwave (HPM) pulse generators for radiating purposes should generate bipolar pulse(s) due to the absence of non-radiated dc and low-frequency components at their frequency spectrum. With respect to the number of generating pulses and corresponding basic structures, HPM pulse generators are divided in two categories:    1. Monocycle (Single Bipolar pulse) Generators    2. Multi-cycle (Train of Bipolar pulses) Generators
The best multi-cycle waveform, especially for radiation, is a sequence of bipolar pulses (rectangular positive and negative sub-pulses) with high peak power of each monocycle and with high energy due to the number of cycles.
From the generator's structure point of view, the best results in pulse forms can be achieved by using voltage charged transmission lines and discharging them by fast switches (spark-gaps or various solid-state switches, including photonically controlled ones).
There are several main criteria that characterize the quality of architecture/circuit of multi-cycle pulse generators:    1. Compactness: acceptance of folded design, etc    2. Number of switches for specified number of pulses and their forms    3. Holding voltage on switches relative to the line's charging voltage(s)    4. Available pulse forms
There are two different categories of multi-cycle generators, based on the principle of operation: Sequentially Switching Generators (SSG) and Frozen-wave Generators (FWG). In a SSG, the switches are closed sequentially with specific time delays depending on the electrical length of the corresponding transmission lines and the required pulse waveform. In a FWG, the switches are closed at the same time and instantly start the discharged process of all transmission lines. This process provides a predominated multi-cycle pulse train on the load.
Most commonly used multi-cycle HPM generators have the same structure for both principles of operation. The main differences are the timing in the closing of all switches and the number of pulses relative to the number of switched lines and associated switches.
FIGS. 1a and 1b illustrate sequentially switched pulse generator with equal pulse's width. For this pulse generator, each line's section produces a positive or negative sub-pulse depending on polarity of the charging voltage. Generated pulses could be with specified different widths depending on electrical length of corresponding lines. Switching time should be changed accordingly. In this generator with sequentially switching, each switch provide generating only one sub-pulse, i.e. for N bipolar pulses 2N switches is required.
For a FWG, the situation is different. FIG. 2a illustrates six-section switchable transmission line FWG having six switches connected in series between sections and one switch connected in parallel to the first section. FIG. 2b illustrates the generated pulse train. Variations of electrical lengths of sections give some freedom in variations of pulses widths and frequency spectrum of generating pulse train. FIG. 3a illustrates a schematic diagram of an exemplary known FWG with six sections and with the switches positioned only in series with the transmission line sections. FIG. 3b shows an ideal five and halve-cycle pulse form on matched load of the generator according to FIG. 3a. 
In the presented multi-cycle generators on FIG. 1a, and FIG. 2a, all switches except for one switch on FIG. 2a, which is connected in parallel to the first transmission line section, should handle double charging voltage. This is a serious limitation for switch selection.
In a few very nearly “Frozen-Wave”—type generators, the number of switches with holding voltages still equal to double the charging voltage(s) could be lower than the number of generating bipolar pulses. However, in these cases the negative effects of switch resistance (Rsw) and switch inductance (Lsw) raise drastically with a decrease in the number of switches.
Modern trends in High Power Microwave (HPM) pulse generators, which can be used for a variety of applications, are directed to increasing power and efficiency as well as the energy density (energy per volume). Pulse generators based on voltage charged transmission lines can achieve some of the best results, especially in the case of generating a series of bipolar pulses (series of cycles), i.e. a pulse train. The coupling of the resulting pulse train to a load, such as an antenna, results in the radiation of a short HPM pulse. This approach has been investigated for over 30 years.
Known multi-cycle microwave generators based on charged transmission lines are not compact due to their schematics/structures. In one group of these generators with acceptable number of switches, holding voltages on these switches are equal to double of charging voltages. Another group of generators with holding voltages on switches that are equal to charging voltages, required many switches (at least two switches per one generating cycle-bipolar pulse). All of these generators are not compact. The main problem consists in developing a simple Multi-Cycle Microwave Generator with charged transmission lines, which is compact, required small amount of switches (not more than the number of generating cycles), and with minimum holding voltages on switches that is equal to charging voltages. Structure(s) should allow generate high-power pulses.
Proud, in U.S. Pat. No. 3,484,619, discusses a SSG arrangement for generating a train of bipolar pulses by sequentially switching oppositely charged transmission line sections by using closing switches, as illustrated schematically in FIGS. 4a and 4b. Zucker et al., in U.S. Pat. No. 5,109,203, also discusses multi-cycle HPM generator structure with various SSG design options and with light activated fast photoconductive closing switches. An additional impedance transformation by non-uniform transmission line is presented. Similar structure is presented by Zucker in U.S. Pat. No. 5,185,586. Remnev, in SU Patent No. 852135, also discusses a similar multi-cycle HPM generator structure with sequentially switching spark-gaps.
All three of these known generators have the same basic circuit (shown in FIG. 4), which suffers from disadvantages, for example: the design is not compact, the required hold-off voltage on each switch is large—equal to double the charging voltage on each transmission line, two switches are required for generating a bipolar pulse (cycle) (i.e., the total number of switches is twice the number of generating cycles), and switches connected in series introduce considerable losses.
Zucker and London, in U.S. Pat. Nos. 7,268,641 and 7,365,615, discuss a SSG arrangement for generating a train of bipolar HPM pulses by sequential switching of closing switches positioned in parallel to transmission line sections. In such a design, the hold-off voltage on each switch is equal only to the charging voltage. This is illustrated schematically in FIG. 5. Moreover, all switches are no longer positioned on the power flow way and, therefore, their losses are not critical and the number of sections is limited mostly by the conductive losses on transmission lines, i.e., by efficiency. Each switch should handle just the charging voltage before discharging, and slightly more during discharging time. This design is not compact, especially in the case of using strip transmission lines and very short pulses (around 1-2 ns). Furthermore, two switches are required for generating one cycle.
London, in U.S. Patent Application Publication No. 2007/0040623, discusses different SSG arrangements for generating a train of bipolar pulses when the switches are also positioned in parallel to transmission line sections. Additionally, there are lower conductive losses in transmission line sections by eliminating conductors with equal magnitude and oppositely directed currents at both sides of oppositely charged transmission line sections. This is illustrated schematically in FIG. 6.
In all known design variations of these sequential switching generators, at least two switches are required for generating one bipolar pulse (mono-cycle). In the generator shown in FIG. 6, four switches in two successive balanced stages provide generation of a single bipolar pulse. With respect to the generator shown in FIG. 5, the number of switches is double and the power/energy of each pulse is also double with the same physical/electrical length of generator. All of these sequential switching generators are also not compact.
Bovino et al., in U.S. Pat. No. 5,153,442, discuss an arrangement for generating a train of bipolar pulses by sequentially switching oppositely charged transmission lines using closing-opened switches between each transmission line and the common load. Again, however, the design is not compact and two switches are required for generating one bipolar pulse (cycle). Additionally, this design requires fast switches with specific opening time.
Multi-cycle HPM generators, which termed Frozen Wave Generators (FWG), have a basic schematic similar to that shown in FIG. 4. In these generators, all switches should be closed simultaneously and each switch provides generation of one bipolar pulse. Proud, in U.S. Pat. No. 4,127,748, discusses a FWG arrangement for generating a train of bipolar pulses by simultaneously closing all switches connected in series between oppositely charged transmission line sections, as illustrated schematically in FIGS. 7a and 7b. Proud and Norman, in IEEE Transactions on Microwave Theory and Techniques, vol. MTT-26, No. 5, March 1978, pp. 137-140, present some details concerning the operation of FWG with light activated semiconductor switches. Many other papers illustrate the operation of specific FWG with photoconductive and other types of switches. The main disadvantages of such implementations of FWG generators include: the design is not compact, the required hold-off voltage on each switch is large—equal to double the charging voltage.
Samsel, in U.S. Pat. No. 2,792,508 discusses an arrangement for generating multi-cycle waves by using two groups of interconnected, oppositely charged transmission line sections, two blocking capacitors, and one closing switch. This design, however, has a complicated, non-compact structure. This design also requires the hold-off voltage on each switch to be equal to double the charging voltage. Furthermore, this design cannot provide a high repetition rate of generating pulses.
Thaxter, in U.S. Pat. No. 5,650,670, discusses an arrangement for generating high power square wave pulses using charged and non-charged transmission line sections and only one fast switch, which should handle only charging voltage. This generator cannot produce a sequential group of bipolar pulses and, therefore, cannot be used for the purpose of effective radiating as known FWG or SSG architectures. Additionally, negative effects of the switch's resistance and inductance on efficiency and pulse shape are increased due to a relatively large charge transfer through a single switch.
Selemir, Ptitsyn et al., in RU Patent No. 2 258 301, discuss an arrangement for generating a multi-cycle wave using pairs of oppositely charged transmission line sections and only one switch. Again, this design suffers from disadvantages, including: non-compact design, deterioration of the pulse shape due to interconnections between cables—especially for frequently required short pulses, deterioration of the efficiency and pulse shape due to the presence of the capacitor, requirement of a high hold-off voltage equal to the double charging voltage, and high negative effects of the switch's resistance and inductance on efficiency and pulse shape due to a relatively large charge transfer through the switch. Furthermore, in order to achieve high power/energy, parallel connected switches are needed.
FIG. 8 illustrates a schematic/design diagram of an exemplary known three-cycle generator with a single switch.
Ptitsyn, Selemir et al., in RU Patent No. 2 313 900, discuss a similar arrangement as in RU Patent No. 2 258 301, in which the capacitor is replaced by an additional switch. This increases efficiency to some extent and improves pulse shape, but decreases reliability. In addition to the disadvantages discussed above, this design is not preferred for HPM applications for the short pulses and with frequently used strip transmission lines.
Gripshover et al., in U.S. Pat. No. 4,491,842 and in a paper entitled “Frozen-Wave Hertzian Generators-Theory and applications” presented at the 2nd International Pulsed Power Conference in 1979, discuss an arrangement with coaxial cables for generating two-cycle high power pulses with tenth's nanosecond width by using a single switch. This design also suffers from disadvantages mentioned above, for example: non-compact designs, interconnections between cables deteriorate the pulse shape—especially for the frequently required high power short (1-2 ns) pulses especially in the case of low-impedance strip transmission lines, the required hold-off voltage on the switch is equal to double the charging voltage, high negative effects of the switch's resistance and inductance on efficiency and pulse shape due to a relatively large charge transfer through the switch (increases with number of pulses), and parallel connected switches typically are necessary.
A short review of current multi-cycle transmission line pulse generators in conjunction with criteria indicated above shows that there are several needs that will be very important for practical needs to combine together in one multi-cycle HPM generator:
Minimum holding voltage on switches, i.e. equal to charging voltages
Minimization of the number of switches (should not exceed the number of cycles)
Generation of a multi-cycle train with different width of individual cycles (bipolar pulses) to provide broad frequency spectrum
The proposed multi-cycle pulse generators with charged/discharged transmission lines obey all of the above mentioned criteria.
The present invention provides solutions to the problems associated with known systems as described above. The present invention provides a compact design for a multi-cycle HPM generator based on voltage charged transmission lines. The present invention also minimizes hold-off voltage on closing switches—equal to the transmission line charging voltage. The present invention also requires a relatively small number of switches. The present invention can provide corrections of the negative effects of the switch's resistance and inductance. The present invention can also provide a balanced (symmetrically positioned) load. The present invention further provides for the production of cycles of various widths to provide the broadband frequency spectrum.